The manufacture of alkali metal salts of alpha-sulfo fatty acid alkyl esters alternatively referred to as sulfonated fatty acid alkyl esters, alkyl ester- sulfonates, etc. ) via neutralization of fatty acid ester sulfonic acids with aqueous caustic is well known. Such ester sulfonates are predominantly used as surfactants in washing and cleansing agents and products.
The known processes for making these ester sulfonates in good yields suffer from the formation of certain impurities including dark-colored impurities, organic and inorganic salts, and disalts. The ester sulfonic acids, from which the alkyl ester sulfonates are derived, are obtained by sulfonating fatty acid esters or, less preferably, by sulfonating and esterifying fatty acids. In order to obtain high sulfonation yields, excess sulfonating agent in combination with greater processing times and/or temperatures is required. These conditions can result in undesirable side reactions including the formation of the dark-colored impurities and the acid forms of organic and inorganic sulfate salts, and disalts. Examples of such sulfonation processes are described in U.S. Pat. Nos. 3,485,856 (Wulff et al.); 4,695,409 (Piorr et al.); and 4,820,451 (Piorr et al.); German Patent Application 3 535 184 (Imamura et al.); Japanese Laid Open Patent Publication Number 290842/90 (Application Number 113423/89); and "The Journal of the American Oil Chemists Society", 52 (1975), pp. 323-329.
For aesthetic and other reasons, dark-colored ester sulfonate compositions are not suitable for use directly in washing or cleansing agents and products. Therefore, the dark ester sulfonate products have heretofore been bleached in order to lighten their color. Typically, the dark products are treated with an aqueous bleaching agent, such as hydrogen peroxide or hypochlorite, before and/or after neutralization. Such bleaching processes are described in U.S. Pat. Nos. 3,159,657 (Wulff et al. ); 3,452,064 (Stein et al. ); 4,547,318 (Kloetzer et al. ); 4,671,900 (Schmid et al.); and 4,874,552 (Richtler et al.).
The art has recognized certain inherent problems in the bleaching process, particularly handling difficulties, hydrolysis of the ester group, and formation of sensitizers. Heretofore, these problems have been dealt with, inter alia, by optimizing the bleaching process itself, or by modifying the ester sulfonation process itself to deliver an ester sulfonate with less color and/or fewer impurities. This allows for the use of milder bleaching conditions which may mitigate the problems associated with bleaching processes. Such processes are described in U.S. Pat. Nos. 3,997,576 (Oghoshi et al. ); 4,080,372 (Stein et al. ); 4,547,318 (Kloetzer et al.); and 4,671,900 (Kloetzer et al.). However, none of these references disclose a process for making ester sulfonate surfactant which is completely satisfactory.
A method of improving the color and purity of potassium and magnesium alkyl ester sulfonate surfactant compositions without the need for bleaching has now been discovered. More specifically, it has been discovered that a lighter-colored, higher purity alkyl ester sulfonate surfactant can be separated from a mixture comprising water and a dark- colored alkyl ester sulfonate surfactant composition. Upon forming a mixture comprising the ester sulfonate surfactant composition and water, the impurities, including dark-colored impurities, are solubilized and the alkyl ester sulfonate surfactant can be separated and recovered from the mixture to yield a product with improved, i.e., lighter, color. The recovered surfactant is also of improved purity, i.e., the recovered surfactant contains a lower level of organic and inorganic salt, disalt, and soap impurities. The process also provides an ester sulfonate surfactant having improved surfactant odor.